The present invention relates to suspension systems and, more particularly, to shock absorber systems for motor vehicles.
Essential in the design of the undercarriage of a motor vehicle is an efficient spring and/or shock absorber system. The shock absorber system should, on one hand, make allowance for road safety while, on the other hand afford the passengers or a shock-sensitive cargo of the vehicle maximally high travel comfort. From the vantage point of the spring and/or shock absorber system, these are conflicting objectives. High travel comfort can be achieved through a maximally soft undercarriage adjustment, whereas with regard to high road safety a maximally hard undercarriage adjustment is desirable.
To resolve this conflict in objectives, a changeover is made from passive undercarriages, which for the most are so far still used, to controllable or active undercarriages. Depending on the predicted usage of the vehicle, the passive undercarriage is configured in its installation either hard ("sporty") or soft ("comfortable"). With these passive systems it is not possible to influence the undercarriage characteristic during the travel operation. In the case of active undercarriages, in contrast, the characteristic of the spring and/or shock absorber system can be influenced during the travel operation contingent upon the state of travel.
In DE-OS 38 27 737, the aforementioned conflict in objectives between road safety and travel comfort is solved in that, whenever road safety permits, a soft undercarriage adjustment provides for high travel comfort. But, if the state of travel of the vehicle is in a more critical range, a harder undercarriage adjustment is chosen. Utilized as evaluation criterion for road safety are wheel load fluctuations during the operation of travel. Wheel load fluctuation is understood as the deviation of the wheel load (normal force between tire and road) from its static value. However, wheel load fluctuation is accessible to direct measurement only with great difficulty, since transducers would need to be arranged between the wheel, or tire, and the road. Measuring the spring travel, in contrast, is relatively simple and can be cost-effectively realized. Spring travel is designated as the shift of the superstructure relative to the wheel. In DE-OS 38 27 737, for example, the spring travel is measured as a substitute variable for the wheel load fluctuation. From these measured values, the sliding effective value and the sliding average value for the substitute variable and its difference are formed. Once this difference has been compared with a predetermined set value, exceeding this predetermined set value triggers an electrical display and/or control signal for control/regulation of the undercarriage by which the latter is set to a harder characteristic.
But a system such as described in DE-OS 38 27 737 provides only coarse and incomplete information on whether the momentary state of travel of the vehicle is critical in terms of safety. Beyond that, an optimal minimization of the wheel load fluctuations is not possible through this system.
German patent application P 41 07 090.9 which is the equivalent of U.S. patent application Ser. No. 07/679,297, filed Apr. 2, 1991 describes a system by which, while enabling an optimal minimization of the wheel load fluctuations, increasing the travel comfort in travel situations that are not critical in terms of safety cannot be optimally solved.
An undercarriage adjustment for comfort can be accomplished, though, in that the adjustable undercarriage is adjusted maximally soft, i.e., the adjustable shock absorbers exercise a slight damping. But a far more efficient control or regulation of the undercarriage, in view of the movements of the vehicle superstructure that determine travel comfort for example, can be accomplished through a so-called "skyhook regulation" such as described in DE-OS 39 18 735.7 and DE-OS 37 38 284.
With the so-called skyhook regulation, the superstructure movements are reduced so as to bring about an improvement of the travel comfort, whereas the road safety is not directly increased. Generally known in undercarriage control, this concept of regulation is based on the model concept of a shock absorber system which attaches to the mass of the vehicle superstructure and is connected to an inertial fix point (skyhook). Such an inertial shock absorber suspension system, not being directly realizable in practice, is, by way of substitution, appropriately activated between the vehicle superstructure and the wheel units.
From a number of publications (Crolla, D. A., Aboul Nour, A.M.A., Proceedings of the Institution of Mechanical Engineers, International Conference of Advanced Suspension, 22-25 Oct. 1988, London or Magolis, D. L., Semi-Active Heave and Pitch Control for Ground Vehicles, Vehicle System Dynamics, 11 (1982), pp. 31-42), in the case of a suspension system featuring shock absorbers whose damping characteristic is adjustable in two stages (hard/soft), known as "semiactive, discrete skyhook damping" which is a switching strategy where the damping characteristic is adjusted contingent on superstructure and spring deflection movements. This strategy is presented in the following table:
______________________________________ Shock absorber Shock absorber in pull state in push state ______________________________________ Va &gt; Vagr hard soft Va &lt; -Vagr soft hard ______________________________________
Here, the superstructure velocity in a vertical direction, at the points of attachment of the suspension system, is abbreviated as Va. Once this velocity exceeds a certain positive bound Vagr (tuning parameter), i.e., as an impetuous upward movement of the car body is taking place, the respective shock absorber is switched in the pull state to the hard characteristic and in the push state to the soft characteristic. Conversely, an impetuous down movement of the superstructure causes in the pull stage a changeover to the soft and in the push stage to the hard characteristic. In the absence of excessive superstructure movements (.linevert split.Va.linevert split..ltoreq.Vagr) the shock absorber operates in its soft tuning, both in the pull and the push stage.
Shock absorbers that are adjustable in their damping characteristic are described, e.g., in DE-OS 33 04 815 and DE-OS 36 44 447.
German patent application P 40 39 629.0 describes a system where, based on the spring deflection movements, so-called "weighted corner velocities" are determined and the superstructure movements are counteracted according to the skyhook strategy described above. The weighted corner velocities are determined in such a way that a specific influencing of the collective superstructure movements, such as heave, pitch and roll movements, becomes possible. Such specific influencing of the collective superstructure movements, e.g., is favorable in steering, braking and acceleration maneuvers of the vehicle, since these movements are particularly stimulated here. The problem underlying the present invention is to design an efficient overall concept for an undercarriage control where particular allowance is made for the aspects of road safety, travel comfort and simple design.